In recent years, there has been a growing demand for the development of so-called highly durable coating media which can form a coating film excellent in durability such as scratch resistance, acid resistance, alkali resistance and solvent resistance from various fields including the exteriors of buildings. Reportedly, the highly durable coating media include a polysiloxane-based coating and a fluoroolefin-based coating. Among other things, the polysiloxane-based coating has been desirably used in various fields because the coating is free of halogen atoms and able to form a coating film excellent in water repellency, in addition to the above-described durability.
A resin composition to be used in the above polysiloxane-based coating includes a curable resin composition containing, for example, a resin obtained by subjecting to condensation reaction a polymer having a hydrolytic silyl group and a functional group other than the hydrolytic silyl group and a polysiloxane having a hydroxyl group (silanol group) bonded to silicon atoms and/or a hydrolytic group (hydrolytic silyl group) bonded to silicon atoms and a compound having a functional group reacting with functional groups contained in the resin as a curing agent. It has been disclosed that this curable resin composition is used to form a coating film excellent in acid resistance, scratch resistance and others (refer to Patent Document 1 and Patent Document 2, for example).
The curable resin composition disclosed in Patent Document 1 and Patent Document 2 forms a cured coating through a hydrolysis-condensation reaction with the resin and the curing agent. This hydrolysis-condensation reaction can take place, for example, at temperatures of 0 to 30° C. However, in order to effect the curing at these temperatures, it is necessary to allow a coating to stand for about one week or so after being coated, resulting in a poor painting efficiency. As a result, for effectively obtaining a cured coating film by using the curable resin composition, for example, as in Patent Document 1, after being coated, an article to be coated is heated at a high temperature of 140° C. However, where a coating containing the curable resin composition is used, for example, it is difficult to heat the coating film after being coated on exterior walls of buildings. It is, therefore, difficult to form a sufficiently cured coating film.
Further, the polysiloxane-based coating now has been evaluated for possible uses other than a coating for the exteriors of buildings, for example, that for vehicle components. These components include bumpers and side view mirrors. Base materials of the components, however, often include plastic materials such as polypropylene and polycarbonate, which are in general vulnerable to heating. Therefore, there is a problem in that where the polysiloxane-based coating described in Patent Document 1, Patent Document 2 or the like is painted and thermally cured, the base materials may deform or discolor.
There is disclosed a coating which contains a siloxane copolymer made up of, for example, a siloxane polymer and a polymer block having a vinyl group on the side chain, as a polysiloxane-based coating which is not required for heating on curing a coating film (for example, refer to Patent Document 3). The coating disclosed in Patent Document 3 will form a cured coating film through radiation of ultraviolet rays or the like after being coated, thereby the coating can be used in an application where it is difficult to carry out heating and curing, for example, as a coating for the exterior of buildings and that for heat-sensitive base materials. However, a coating film obtained by coating such coating media that contain the siloxane copolymer disclosed in Patent Document 3 is not sufficient in weatherability such as scratch resistance, acid resistance, alkali resistance and solvent resistance, as compared with a coating film obtained by using a conventional coating high in weatherability (for example, the coating disclosed in Patent Document 1, Patent Document 2 or others).
Incidentally, regarding highly durable coating media, the coating media based on organic solvents are now being replaced by those based on aqueous solvents in view of global environmental protection. Polysiloxane-based aqueous coating media include, for example, a curable resin composition containing a composite resin constituted with a polysiloxane segment and a polymer segment having a hydrophilic group and a compound having a functional group reacting with a functional group contained in the resin as a curing agent. It is reported that this curable resin composition is able to form a coating film excellent in storage stability, room-temperature curing, gloss retention and durability (for example, refer to Patent Document 4).
However, the curable resin composition disclosed in Patent Document 4 is cured through a hydrolysis-condensation reaction of the composite resin and the curing agent. Therefore, in order to obtain excellent physical properties of coating film as described above, it is necessary to provide a step where a coating is allowed to stand for curing for about one week at room temperature after being coated as in Patent Document 1 or Patent Document 2, resulting in a poor painting efficiency. As means for improving the efficiency of painting work, there is proposed a method for heating a coating at a high temperature after the coating containing a curable resin composition is coated, thereby promoting the formation of a cured coating. For example, where a coating containing the curable resin composition is used for the exteriors of buildings, it is difficult to heat the coating film after being coated on the exteriors of buildings. Thus, it is difficult to form a fully cured coating film.
Patent Document 1: International Publication No. WO/96-035755 Pamphlet
Patent Document 2: Japanese Published Unexamined Patent Application No. 9-025455
Patent Document 3: Japanese Published Unexamined Patent Application No. 2000-34326
Patent Document 4: Japanese Published Unexamined Patent Application No. 11-279408]